The invention is based on a cooktop and a method for operating such a cooktop.
Cooktops with a number of heating elements combined to form a heating zone are known from the prior art. The heating elements can be disposed concentrically for example and be operated so that outer rings of the heating zone are only activated when the pot diameter of a pot placed on the heating zone exceeds a limit value. In other known cooktops many heating elements are disposed in a matrix or grid, for example a 4×4, 6×6 or 8×8 grid. A cooking utensil element placed on the cooktop is detected and a number of heating elements disposed beneath the base of the cooking utensil element are combined to form a freely configurable heating zone.
The heating elements combined to form a heating zone are each supplied with a heating current by a power supply unit. A power supply unit configured as an inverter is frequently assigned to each inductor, particularly when the heating elements are induction heating units or inductors, it being possible for the inductor to be operated independently of the other inductors by way of the power supply unit. A control unit comprises means or suitable driver units for determining a characteristic variable of the heating current, for example an amplitude, frequency, direct current voltage or phase. The control unit further comprises means or suitable interfaces or driver units for actuating the switching unit as a function of a selected power level. The power level can be selected by the user by way of a user interface.
In the field of induction cooktops in particular the problem may arise during the operation of a number of combined heating elements that the different induction heating elements have to be operated simultaneously at the same frequency to avoid intermodulation hum. In induction cooktops the heating power is generally determined by varying the frequency of the heating current, so that synchronized operation of the induction heating units at the same time renders independent determination of the heating powers of the two heating elements impossible. It is further known to operate a number of induction heating elements of an induction cooktop, which are assigned to different heating zones, at different frequencies and to ensure that the difference between the frequencies of the heating currents does not drop below a minimum value determined by the audibility threshold of the human ear, to avoid audible hum. Particularly sensitive people and children can still hear the hum in some circumstances and feel disturbed by it.
In the field of cooktops with radiant heating units it is further known to switch the heating elements on and off periodically to achieve a specified average heating power over a heating period. A ratio of the duration of the phases in which a heating element is switched on to the overall duration of the heating period then corresponds to the ratio of setpoint heating power to a maximum heating power.
The simultaneous switching on and off of a number of heating elements can lead to a problem with flicker, in other words a feedback from the cooktop to the household power network, both in induction cooktops and in cooktops with radiant heating units. The flicker can produce voltage peaks which briefly overload the household power network, even if the maximum power of the household power network is not yet fully utilized away from the voltage peaks.